This invention relates to devices for controlling loudness and, more particularly, to a new and improved apparatus for automatically providing a uniform average loudness level of broadcasting and like sound signals.
Commonly assigned U.S. Pat. No. 3,594,506 entitled "Loudness Level Indicator," the disclosure of which is hereby incorporated herein by reference, describes a loudness level indicator for accurately monitoring audio loudness levels, which includes an equalization network to which broadcasting and like signals are applied, the equalization network having a transfer characteristic which is inversely proportional to a 70 phon equal loudness contour, representative of the loudness levels of sounds as a function of frequency for sound pressure levels ranging from about 60 db to about 80 db. The system includes eight octave band filters which separate the equalized signals into frequency bands covering the entire audio range, which because of the equalization network provide bands of sound of equal loudness as output signals having equal amplitudes. The separated signals are rectified and linearized and then combined linearly in a signal combining network. The combined signal is applied through a ballistics compensation network to an indicating instrument which provides a visual indication of the amplitude of the combined signal.
The loudness research reported in the aforementioned patent and the experience gained in the design of the loudness level indicator were embodied in a system described in commonly assigned U.S. Pat. No. 3,582,964 entitled "Automatic Loudness Controller," the disclosure of which is hereby incorporated herein by reference, for automatically controlling the loudness levels of broadcast sounds. This device, which has been used with success for over a decade, and was designed to be used as the final variable gain device in an audio broadcast chain (i.e., at the transmitter) is solely a loudness limiter; that is, it reduces the level of the signal if it senses that the signal going through, although at correct modulation, is possibly too loud. The device is not capable of raising the signal level and consequently cannot affect the carefully controlled limits on the 100% modulation.
Experience with this earlier automatic loudness controller has shown that control of loudness might better be done in the broadcast studio than at the transmitter, at or near the end of the audio broadcast chain; that is, it is more convenient to establish correct loudness levels in the studio and then transmit the processed signal by telephone line or microwave link to the transmitter site. However, since the broadcast chain traditionally includes a peak limiter as the final element, even if correct loudness levels were established at the studio, this peak limiter might very well defeat these carefully established loudness levels, particularly if low frequency signals were brought up to be equally loud to earlier-processed high frequency signals; that is, if the loudness controller tried to raise the loudness level of an essentially low frequency passage so as to have a loudness equal to previous program material consisting essentially of higher frequencies, the peak limiter would recognize a potential for overmodulation and reduce the level of the low frequency passage, and thus effectively defeat the action of the loudness controller. This result is satisfactory in the sense that compliance with FCC standards is assured, but the operator loses aesthetic control over the signal ultimately broadcast because of another automatic device (i.e., the peak limiter) which operates contrary to the intended purpose of the loudness controller. It would be much better to have all of the control functions embodied in one device capable of analyzing these opposing criteria, namely, loudness level and signal level, and making a single gain-adjustment decision; that is, if analysis of the potential loudness of the signal indicates a possibility of its over modulating, if the gain of the signal is controlled on the basis of signal level rather than loudness the possibility of the final peak limiter being called upon to limit the signal before it is broadcast would be essentially eliminated.